This invention relates to the preparation of potassium ferrate (VI), K.sub.2 FeO.sub.4, by a process in which the K.sub.2 FeO.sub.4 is crystallized from concentrated aqueous potassium hydroxide (KOH) solution, separated therefrom, and washed to remove the residual KOH solution from the separated KOH crystals.
In the preparation of potassium ferrate (VI), K.sub.2 FeO.sub.4, the product is recovered by crystallization from a concentrated potassium hydroxide solution, for example, from a near saturated (viz. 50%) aqueous KOH solution. This very concentrated KOH solution clings very stubbornly to the K.sub.2 FeO.sub.4 crystals and cannot be completely removed by either filtration or centrifugation. Typically, the raw K.sub.2 FeO.sub.4 product still contains 20 to 30% adherent KOH. This alkali hydroxide content is undesirable because it interferes with the drying of the material and also causes problems in such uses of K.sub.2 FeO.sub.4 where an excess of alkali is harmful.
The recommended laboratory method of KOH removal involves washing of the raw K.sub.2 FeO.sub.4 with a succession of several different organic solvents: benzene, followed by a lower aliphatic alcohol (viz. methanol, ethanol, isopropanol), and finally by diethyl ether. This standard approach is described in Jones, Inorganic Synthesis, Vol. 4, pp. 164-168, (McGraw Hill, 1953). In the customary washing, sequence of benzene-alcohol-ether, the benzene is used to eliminate the water present in the strong KOH solution because the presence of water is believed to enhance an undesirable oxidative reaction between ferrate VI and alcohol. The alcohol used in the next step is the means to remove the adherent KOH. Alcohol dissolves the potassium hydroxide but leaves the K.sub.2 FeO.sub.4 on the filter. The ether used in the final step serves the purpose of removing the remaining alcohol (present from the preceding step), and also gives a practically dry finished product because the ether can be evaporated at room temperature in a desiccator.
Although potassium ferrate (VI) is one of the best oxidizing agents known, and potentially has many desirable applications as an oxidizing agent, it is not as yet been produced or used commercially on a large scale. One of the unsolved problems connected with the commercial production of K.sub.2 FeO.sub.4 is a need for a better procedure to remove KOH from the crystallized K.sub.2 FeO.sub.4. The use of a sequence of washes with organic solvents is undesirable for commercial practice. Such solvents are inherently flamable and may provide toxicity problems. Further, the process can be hazardous in that the ferrate may oxidize the organic solvent, and such oxidation may be so rapid as to result in an explosion. For example, alcohols such as methanol or ethanol are subject to ferrate oxidation, especially in the presence of water. Therefore, if the water is incompletely removed from the crystallized ferrate by the preliminary benzene wash, a potential process hazard may exist. Further, although the potassium hydroxide is selectedly soluble in the alcohol, it is not sufficiently volatile at a low temperature to permit drying of the ferrate without decomposition. Therefore, it is apparently necessary to use a further wash with a more volatile solvent, such as diethyl ether, to remove the alcohol, so that the residual organic solvent can be removed in low temperature drying of the ferrate product. If residual organic solvent or other organic matter is accidently allowed to stay with the ferrate during and after drying, violent explosions can occur.